Jeff Lewatowski trudged upstream, the chilly, knee-deep Gunpowder River flowing around his waterproof overalls. Following the inside curve of a bend, he picked a likely looking spot and settled in, much like the trout he planned to catch.

"They want to take advantage of the bugs," said Lewatowski, 39, who lives in Havre de Grace and guides fishing trips around the state. "They get very excitable the more bugs there are on the water. … They compete in a spot in the creek to sit."

May through June is sulphur mayfly season, with thousands of mayfly nymphs swimming up from sunken logs and rocks to molt and flutter away for their brief adult lives. After they mate and the females lay eggs, the flies fall back onto the water and die. The upper Gunpowder's brown trout snap up the insects at any stage of their life, and fishermen add flies of their own to the mix, in hopes of fooling the predators.

A few trout were rising around Lewatowski, breaking the surface to grab a meal. But catching them is no easy task.

"Trout are some of the wariest of all fish," Lewatowski said. "They're the Ph.D.s of the fish world. … They're very wise to the way the natural flies look, so you really have to be stealthy on your approach."

That means walking slowly, speaking quietly and making the bait look as authentic as possible.

Backwater Angler, the Monkton fly shop that supplies Lewatowski with some of his clients, stocks dozens of types of flies tied by locals and commercial producers. Each fly is a hook wrapped in feathers, string and hair, fashioned to imitate a mayfly, caddisfly, midge or other insect.

With sulphur mayflies the most common, Lewatowski "matched the hatch," bringing submergible wet flies that look like the "emerger," or rising nymph, and dry flies such as the adult "dun" or the dying "spinner that ride on top of the water.

He tied his first fly, a Cul De Canard Comparadun, to the end of the monofilament line and whipped his 81/2-foot graphite rod to cast the fly onto the surface of the riffle, a fast-flowing stretch where many mayfly nymphs tend to hatch into adults.

"In fly-fishing, the weight of the line carries the fly to the fish," Lewatowski explained. "We want the fly going downstream as natural as possible," with slack on the line so that the current carries it along with the real mayflies.

"Each of our casts is a measured cast," he said, watching as he passed his fly over a trout's lie. "You have to really present it perfectly for him to come eat it."

The day was just about perfect, Lewatowski said, with a strong hatch and rainy weather making the anglers less visible to the trout. But the fish were still resistant.

"These fish have seen some flies," he added as trout by the far bank struck at real bugs and he turned to cast his after another fruitless pass.

"They're giving you the finger right there," said Jennie Lindqvist, who was having the same luck 20 feet downriver. "The fin."

Lindqvist, 38, a Monkton resident and manager at Backwater Angler, comes to the Gunpowder to fish, often with Lewatowski and Theaux Le Gardeur, the shop's owner — though "not enough and not as often as I'd like."

"We work together with Theaux, two of the other guys that are in the area, and it's usually just … 'Let's go fishing after work,' " she said. "It's kind of like, 'Let's go out for a beer' for us."

Lindqvist learned to fish for trout, grayling, salmon and pike in Finland before moving to the United States in 2001. Compared with Finnish lakes and rivers, the Gunpowder is a small creek, but never a dull one.

"Every spot is different each time," Lindqvist said. "The hatch, it changes, the fish positioning changes, and the depth of the water. … There's always some factor that's going to change it a little bit."

Lindqvist and Lewatowski said the best part of fly-fishing is the opportunity to enjoy the outdoors.

"Trout are one fish species that always seem to live in the most beautiful places," Lewatowski said. "They live where there's clean, cold water, and there's not a lot of places where there are that. … Usually, what's good for the trout is good for the environment."

The upper Gunpowder is a blue-ribbon trout fishery created by the cold tailwaters of Prettyboy Dam. While other species thrive downstream, closer to the warmer waters of the Loch Raven reservoir, cold outflow from the bottom of Prettyboy helps keep the northern stretch at about 60 degrees, the best temperature for wild brown trout.

The state Fisheries Service stocks the section above Loch Raven with trout, mostly from the Albert Powell Hatchery in Hagerstown. But the brown trout are a self-sustaining population introduced in the 1980s, and respond more naturally than the farmed fish to flies. Lewatowski and Lindqvist fish in a catch-and-return section, using barbless hooks to avoid killing the trout they catch.

"We choose to catch and release to help preserve the resource," Lewatowski said. "By practicing-catch-and-release, we leave the trout in the creek for everyone to enjoy."

Finally, Lewatowski's line jerked taut and his rod bowed as a fish took his fly. Calmly but quickly, he started to pull in the line by hand, coiling it at his side while the trout swung downstream. A minute later, he had the fish, an olive-colored 11-incher with red spots and a gold belly, gasping in his hands.

"The visual take of the dry fly is the coolest thing in fly-fishing," Lewatowski said. "You get to watch the trout take your fly."

A minute later, he unhooked the trout and let it slip free.

"No matter what kind of fish you're catching, if that rod's bent and the line's tight, you're having a good time," he said.